Super strength steel alloy composition and product and process of preparing it



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SUPER STRENGTH STEEL ALLOY COMPOSITION AND PRODUCT AND PROCESS OFPREPARING IT Filed Sept. 21. 1961 3 Sheets-Sheet 1 Tic. l.

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SUPER STRENGTH STEEL ALLOY COMPOSITION AND PRODUCT AND PROCESS OFPREPARING IT Filed Sept. 21. 1961 3 Sheets-Sheet 3 INVENTOR. JZMES 742mrs/e AT TC'EA/EV United States Patent 3 198,630 SUPER STRENGTH STEELALLOY COMPOSI- TION AND PRODUCT AND PROCESS OF PREPARlNG IT James P.Tarwater, Parma, Ohio, assignor to Republic Steel Corporation,Cleveland, Ohio, a corporation of New Jersey Filed Sept. 21, 1961, Ser.No. 140,072 12 Claims. (Cl. 75-124) This invention relates to superstrength steel alloy composition and product and process of preparingit. More particularly the present invention relates to a steel alloycomposition capable of being treated to produce a super strength body asone having a tensile strength substantially above 300,000 p.s.i., whilehaving a substantially definite yield point which is substantially thesame as the ultimate tensile strength and further, while having a highdegree of ductility.

The invention further relates to the provision of an alloy steelcomposition of the super strength type, which will have a very high as-tempered strength attained over a substantial range of temperingtemperatures, as from about 400 F. to about 900 R, such compositionpreferably including cobalt and/ or aluminum.

The presentinvention further provides a super strength steel alloycomposition which will have high fracture toughness as measured by theresistance of a notched sample' of sheet material to crack propagationunder stress.

Super strength steels have now become a recognized group of steelalloys, so that various compositions have been disclosed by differentmanufacturers of such steels along with their claimed tensile strengths,which in practically all instances are substantially less than 300,000p.s.i. In most instances these steels are in the medium carbon range andinclude such other alloying elements as manganese, silicon, chromium,molybdenum, vanadium and nickel. In practically all instances, however,the nickel content has been quite low, rarely above 2%; and some otherof the elements aforesaid have been present in amounts which aresubstantially above those contemplated as tolerable in accordance withthe present invention, for example, chromium has usually beensubstantially higher than the upper limit given in the present inventionof about 0.5% The present invention by contrast with these may be termeda medium carbon nickle steel wherein the nickel present is from about 3to 7% and for certain purposes, further limitation as hereinafter setforth are imposed upon the composition.

The prior art, for example, has also suggested a number of suchcompositions of what are referred to generally in the art as Ladish-typesteels, certain of these being set out in U.S. Patent Nos. 2,919,188 and2,921,849, both owned by Ladish Company, Cudahy, Wisconsin. These steelshave a quite low nickel content, it being stated, for example, in thefirst of these patents:

The nickel content is always less than the chromium content and isalways less than the molybdenum conten As hereinafter stated in greaterdetail, the compositions of the present invention attain results oftensile strength, yield strength and ductility which are substantiallysuperior to the Ladish-type steels and to any other known group or typeof steels available in accordance with the prior art teachings, bothfrom the point of view of the as-tempered characteristics of yieldstrength and tensile strengths over a substantial range of temperingtemperatures and also from the point of view of ductility accompanyingthe high yield strength and high tensile strength characteristics.Another group of these steels all within the general scope hereinaboveoutlined has greatly superior fracture toughness as contrasted withknown super strength steels, which is a characteristic that is becomingmore and more important in some of the uses for which such steels arerequired today. One such use of prestrained and strainaged materials isin the making of high velocity rotors such as are required for highspeed precision pumps where close tolerances require a very high 0.02%offset yield strength. Other uses include but are not limited to tensilemembers of rigging, frames, structures, particularly for aircraft andmissiles where high strength-toweight ratios are required.

The prior art has also suggested a process of treating steel which is insome respects essentially similar to the process herein disclosed;except that this process has here-.

tofore been used only on steels having low nickel content such, forexample, as AISI Type 4340 steel (1.74% nickel) and where the chromiumcontent was substantially higher than that as desired or contemplated inaccordance with the present invention (i.e. 0.82% chromium); ThisAISI-Type 4340 steel and the process of treating it has been describedin considerable detail in the Transactions of the American Society ofMetals Quarterly Edition for March 1961, pages 72-83. The fact that theresulting steel, while having super strength in'tension, was fatallydeficient in its ductility characteristics is set out on page 82 of thisarticle wherein it is stated the strength is accompanied by an almostcomplete loss of stable elongation.

In contrast with this, the steels of the present invention mayadvantageously be treated in accordance with this process, i.e. treatingby austenitizing at a sufiiciently high temperature, then quenching,then tempering at a relatively low temperature, followed by a plasticprestrain beyond the elastic limit of the metal, and a subsequenttreatment which is herein referred to as strain-aging in which the steelpreviously subjected to strain as afore-. said, is held at a desiredtemperature for a period of time such that the desired characteristicsare induced and substantially permanently maintained, therein.

When a steel alloy composition within the limits set on in the presentinvention is treated by the process aforesaid, the resulting steel bodywill have super strength in the direction in which it was prestrained,accompanied by a yield point which is reasonably definite and with a0.2% offset yield point as hereinafter defined, which is approximatelyequal to the tensile strength. Such a steel product is per se a part ofthe present invention. This product, however, cannot be distinguishedfrom other steels which do not share the same physical characteristicsor desirable properties by any of the conventional tests. As such,therefore, this product can be defined, asfar as can presently be known,only by the process of making it, coupled with the composition thereof,both of which are necessary in order that the product shall have all thenovel and desirable physical characteristics of the present prodnet.

The invention will be better appreciated from a detailed descriptionthereof which follows, and wherein reference is made to the accompanyingdrawings, in which:

FIG. 1 is a comparison of a steel which has been tempered, but notprestrained or strain-aged as against a steel which is treatedcompletely in accordance with the present invention to include elevatedtemperature strain-aging, the figure being a chart of tensile stressagainsttstrain and also showing the 0.02% offset lineand the 0.2% olfsetline;

FIG. 2 is a plot of yield strength (0.2% offset) in units. of 1000p.s.i. against tempering temperature in degrees Fahrenheit; 1

FIG. 3 is a chart similar to FIG. 2 and for the same set of test samplesof tensile strength against tempering tempe'rature; and p FIG. 4 is aview of a test piece as used for determining the fracture toughness of asample of steel by determining the strength level at which a crackpropagates rapidly in a sharply notched sample.

From a broad point of view, a group of steels within the general limitsas set out will accommodate themselves to three different but alliedobjects and purposes. All these purposes require a super strength steel,i.e. one with a very high as-tempered or as-heat-treated tensilestrength. Many high strength steels do not have a true yield point, sothat it has become a custom to get a so-called yield point by firstplotting stress applied to a test piece, for instance, stress intension, against the actual elongation or strain on this test piece.During the period of application of force (stress) below and up to theelastic limit of the material, such a plot is a relatively straight lineinclined upwardly and to the right. Beyond a point substantiallycorresponding to the elastic limit of the metal the plot curves off tothe right with no perceptible sharp break as a curve which is initiallysubstantially tangent to the previous straight line. For this reason ithas become common to draw an arbitrary straight line parallel to thestraight portion of the stress-strain plot that lies below the elasticlimit, and which is offset therefrom by 0.2% in strain or elongation andto take the point of intersection of this arbitrary line with theprincipal curve itself as the yield point or rather as the 0.2% offsetyield strength. The present invention, however, provides steels, whichhave after treatment substantially a true and quite sharp yield point asshown by the comparison of curves A and B on FIG. 1 of the drawings.

The broad composition of steels within the present invention will firstbe discussed. This composition is as follows:

About 0.350.55% carbon About 3-7% nickel About 02-05% chromium About 2%manganese About 0-2% silicon About 0-0.5% molybdenum About 00.2%vanadium About 05% cobalt About 01% aluminum v and not over about 0.01%each of sulfur and phosphorus and the remainder being iron withincidental impurities.

Of the ingredients hereinabove listed, the carbon is selected in aso-called medium range of 0.35-0.60%; as steels having a lower carboncontent than that in the range selected present no real advantage overthe prior art; while steels having a higher carbon content than therange selected are embrittled, so that the desired characteristics ofductility are not present. The preferred range of carbon from the pointof view of providing a steel having a maximum ability to produce adesired prestrained and strain-aged body (to give super strength plusductility) is somewhat narrower in that for this purpose the carboncontent is preferably about OAS-0.50%. It is further noted that thispreferred range carbon content is somewhat higher than that of theAISI-Type 4340 steel (which has 0.40% carbon); and yet the resultantsteel alloy has a much higher ductility as treated in accordance withthis invention.

The next most important alloying ingredient in the steels of the presentinvention is nickel. From a broad point of view of this nickel contentmay be from about 3 to about 7%. From a more limited point of view, itis preferred that nickel shall be from about 3 to about excellentresults having been obtained at both these limits and no reason beingknown why the range therebetween should not give equally excellentresults for many though not all purposes. The lower limit of nickelcontent, however, is quite critical in that when the amount of nickelpresent is substantially below 3%, such as in the AISI- type 4340 steelwherein there is a nickel content of only 1.74%, there is very lowductility for the prestrained and strain-aged bodies. The higher limitfor nickel is not as critical, but substantially higher ranges of nickelgive essentially different type alloys, which do not follow generallythe rules nor have the characteristics applicable to the present groupof alloy steels. It is also to be remembered that nickel is much moreexpensive than are some of the other ingredients, particularly the ironingredient which is of course present to a very major extent and,therefore, as the percentage of nickel is greatly increased, the cost ofthe final alloy steel is correspondingly increased.

Manganese is similar in some of the characteristics provided thereby tosilicon, in that both provide some degree of hardenability for the steelalloy. Generally, a residual of manganese is maintained to combine withsulfur so as to prevent hot workability difficulties. However, with ajudicious selection of raw materials, the manganese additions may bereduced or wholly omitted, so that the lower limit may be said to bezero. The maximum of about 2% is chosen, as there is no apparentimprovement in the characteristics of the products with greater amountsof manganese. Thus the upper limit is not a critical limit, but is onedictated to the maximum extent at least by economic factors, rather thanby factors having to do with the technical properties of the product.

Silicon is generally found in many steels to some extent and hasgenerally the function of retarding the tempering reaction at temperingtemperatures of 600 and less. Generally, silicon is added to combinewith oxygen in the melt, however, with special melting techniques, thesilicon may be wholly omitted, so that the lower limit may be said to bezero. The maximum value of silicon of about 2% is chosen for the reasonthat as the amount of silicon is increased, the final product tends tobecome more and more brittle. Values greater than about 2% thus impartundesired brittleness to the product.

Chromium tends to prevent graphitizing during the heat treating or inservice of the steel alloy bodies and is preferably present in theamounts from about 02-05% in accordance with this invention. Thepreferred concentration range is from about 0.25% to about 0.35%.

It is desired that sulfur and phosphorus be minimized, as it is wellknown in all ferrous metallurgy that sulfur tends to render the partsmade therefrom brittle when hot, while phosphorus make them brittle whencold. The values for sulfur and phosphorus, therefore, are given asmaximum tolerable values throughout this specification; as it isunderstood that the lowest possible values are desirable, but that it isnot practically possible to eliminate these elements altogether undercommercial operating conditions.

Another element which is optionally usable in the composition is cobalt,the outside limits of such use in accordance with the present inventionbeing about 0-5%. Thus it is specifically contemplated that compositionshaving no cobalt at all are to be considered as included in the presentinvention; while compositions over about 5% are to be considered asexcluded. The upper limit in this case is not particularly critical. Thefunction of cobalt, at least in the presence of some silicon, is toimprove the temper resistance and provide desirable physicalcharacteristics in the material particularly on an astempered basis; inother words, without the steps hereinafter discussed of prestraining andstrain-aging. On the other hand, as hereinafter set out,cobalt-containing alloys have been proven not only useful and operative,but highly desirable when prestrained and strain aged.

Aluminum is another optionally usable element, and is desirable for useparticularly in steels containing cobalt as hereinafter set out. As faras is known this element acts as a deoxidizer for the steel compositionsin the relatively small concentrations contemplated in accordance withthe present invention, i.e. from 0-l%. In this instance also the 0 ismeaningful in that it is specifically contemplated that many steel alloycompositions in accordance with this invention may not contain anyaluminum whatsoever.

Molybdenum and vanadium are also optionally present elements in thatthey may be absent altogether, which is the reason that the lower limitin each instance as to these elements is given as 0. The maximum may betaken as about 0.5% for molybdenum and about 0.2% for vanadium.

It has been found generally that the elements chromium, molybdenum,vanadium, tungsten and columbium may be collectively termed carbideformers. They, or some of them, are used in practically all highstrength steels to some extent. It is noted, however, that substantialquantities of members of this group of metals tend to render the steelhard and strong, but with very little ductility. In general they areused in steels, which are to be tempered at above 600 F., and therefore,above the range where super strength steels usually exhibit theirmaximum tensile strength. In the case of most of the steels inaccordance with the present invention these elements are kept at or neara minimum, consistent with the desire for strength, and in order thatthe resulting alloy steel shall have substantial ductility.

The steels of the present invention are characterized more predominantlyby the presence of the non-carbide formers such as nickel, silicon,cobalt and aluminum. It has been found that manganese has some of thecharacteristics of the carbide formers and some of the noncarbideformers, so that it cannot be classed exclusively with either group.

Other elements may be present in relatively small or trace amounts, suchas calcium, copper, titanium zirconium, columbium, tantalum and boron.However, the amount of any of these metals, if present in the alloy, orthe total of all of them, is so small as not substantially to affect theproperties of the alloy as a whole. As such, these alloys can all beclassed as incidental impurities in the iron if and to the extend thatthey are present at all. It is not a part of the present inventionintentionally to introduce any of these elements as such.

It is contemplated that most if not all metal parts according to thepresent invention will be heat treated at least at first by a more orless conventional heat treating procedure which will include first anaustenitizing step in which the steel alloy is first heated to and heldat a temperature preferably in excess of the range of about 14001450 F.for a period of time sufiicient to bring the metal to a relativelyuniform and stable condition at this temperature. The metal is thenquenched in oil or in a fused salt bath as hereinafter particularlynoted, this quenching being entirely conventional and hence not beingdescribed in any greater detail. Thereafter the quenched body is usuallytempered by bringing it to and holding it at a selected temperingtemperature which is usually in the range of about 350 to about 600,although in some instances tempering temperatures as high as 800 or moremay be used. It is found, however, that for maximum strength, the lowertempering temperatures are usually desirable, with a maximum usually ofnot over 600 F. and with a preferred tempering temperature for manyalloys according to the present invention of about 400", alltemperatureshere given being in degrees Fahrenheit.

The usual experience with previously known steels, such as the AISI-Type4150, is that as the tempering temperature is raised, the yield strengthand the ultimate tensile strength is progressively reduced. Data as to4150-type steel is shown by the dotted lines G and G in FIGS. 2 and 3respectively. The steels according to the present invention, however,have increasing yield strengths (0.2% oifset points) with increasingtempering temperatures from 400 to 600 and even above 600, the-yieldstrengths of preferred compositions are above those for conventionalsteels such as the 4150 type. This is illustrated best in FIG. 2 of thedrawings showing in graphic form results of testing preferred composi- 6tions according to the present invention with respect to type 4150steel.

The test results are represented by lines C and D on FIG. 2 of thedrawings, the composition of the material tested to produce line Ccontaining both cobalt and aluminum and the composition of the materialtested to produce line D containing cobalt, but no aluminum, both ashereinafter set out in detail. There is also shown on this same figure aline E representing similar data for a composition similar to that usedfor the test forming the basis for lines C and D, but in this casecontaining neither cobalt nor aluminum. These are further to be comparedwith a composition forming the basis for curve P, which not onlycontained no cobalt nor aluminum, but did contain both molybdenum andvanadium, thus having an excess of carbide formers in accordance withthe preferred compositions of this invention. The composition formed onthe basis of line F is relatively undesirable from the point of view ofits as-tempered characteristics, which are those shown in FIGS. 2 and 3.Thus this composition, while being within the invention in that it isadvantageously usable with the specific process of the present inventionincluding prestraining and strain aging, does not have the desirablecharacteristics necessary for another phase of the invention having todo with a relatively high as-tempered strength and yield strength over asubstantial range of tempering temperatures. All these may further becompared with line G on FIG. 2, which is that for a previously knowntype of steel, namely, No. 4150 steel. For purposes of record, inaccordance with a standard reference book on steel, the composition ofNo. 4150 steel is as follows:

Carbon0.480.53%

Manganese--0.75-1.00%

Sulfur and ph0sph0rus-0.40% maximum each Silicon0.20 0.35

Nickel Chromium-0.80l.l0% Molybdenum--0.l50.25%

with the remainder being iron with incidental impuities.

In FIG. 3 the lines C, D, E, F and G are drawn with data from stressversus strain tests on the same group of steels respectively as thecorrespondingly numbered lines on FIG. 2 without the prime marks.

From the data illustrated in FIGS. 2 and 3 it is obvione that while thetensile strengths of the several steels tempered at 400 F. is higherthan those tempered at higher temperatures, the steels of the presentinvention exhibit an unexpected improvement or increase in the 0.2%oifset yield strength when tempered up to 600 and higher with respect toprior art steels such as that tested to give the dotted lines G and G.

Continuing now as to the process of the present invention, applicable toa relatively large group of steels all within the present invention, andfollowing the usual tempering step, there are provided prestraining andstrain aging steps. These will now be discussed.

When it is known that a particular part as a steel piece is to have towithstand a particular type of externally applied force such as tension,compression, twisting or' torque in a right hand direction or in a lefthand direction, but not more than one of these four kinds of force(considering right and left hand torque as two kinds), then it ispossible by the process of thepresent invention to attain super strengthcharacteristics in the desired one of these four directions. Inasmuch astension is usually considered as a prime method of testing and manysteel parts must be made to withstand tensile forces as distinguishedfrom either compression, right hand torsion or left hand torsion, thenthe part in question is prestrained in tension in the same manner thatit is desired to be strong in service. Thus a part which is to withstandtension is prestrained in tension, and the part to withstand compressionis prestrained compression, etc. This is necessary in order to avoid theso-called Bauschinger effect. A very general summary of this effect isthat while a part which is to withstand tension, for example, may beprestrained in tension and then strain aged; if this part is towithstand compression, prestraining and strain aging in tension is notof significant assistance. Similarly, if a part is to withstand righthand torsion, prestraining and strain aging in left hand torsion is ofno assistance, and in fact, may even render the part so treated weakerthan a wholly untreated part. With this idea in mind, the prestrainingand strain aging in accordance with the present invention must be donein the direction in which it is desired that the body shall have superstrength.

The term in the direction as so used is intended to distinguish not onlybetween compression and tension on the one hand, and also between righthand and left hand torsion, but also between end-wise force (i.e.compression or tension) on the one hand, and torsion in either directionon the other. This term is used in this manner throughout thisapplication and in the appended claims.

The prestraining in accordance with the present invention is furtherintended to be restricted to a plastic prestrain, i.e. the applicationof sufficient force so as to effect a strain beyond the elastic limit ofthe material, so that there will be a permanent deformation in the bodydue to and following the prestrain step when the applied force isrelieved. This permanent deformation should be of the order of magnitudeof about 1 to 6% of the original dimension of the part in question inthe direction of the strain and as a permanent strain or deformation tothis extent. It can be applied by any suitable apparatus having thenecessary strength and gripping means to apply the force in question inthe desired direction.

If a body he merely prestrained (without strain aging), for example, intension in accordance with the teachings herein given and be testedimmediately thereafter in tension, the new .2% offset yield strengthapproximates the stress level at which the prestrain was terminated. If,however, a sufficient time is left following the act of prestraining togive what is known as strain aging, then the desirable effects of theprestraining will be present. This time and the temperatures at whichthe strain aging is accomplished again are not exactly definite. Thestrain aging apparently takes place much more rapidly as the temperatureis raised and hence is preferably done at an elevated temperature, eventhough it is theoretically possible to effect stain aging at roomtemperature if sufficient time is provided. However, due to the desireto secure the results to be attained in a reasonable and limited amountof time, it is ordinarily preferred to use an elevated temperature forstrain aging plus a sufficient time. This elevated temperature, however,should not exceed the prior tempering temperature used on the same bodywithout undesired results, in effect eliminating all the desirableresults which are sought incident to prestraining and strain agingcombined. The elevated temperature for strain aging is preferably about50 P. less than the tempering temperature. This 50, however, is notnarrowly critical, it being important merely that the strain agingtemperature be somewhat and prefersbly substantially less than thetempering temperature. It has been found that a 50 difference is apreferred differential in this respect. At temperatures of about 50 lessthan the tempering temperature, strain aging can occur to a satisfactorydegree in about two hours, so that increased time beyond two hours doesnot result in any substantial improvement in the results attained.Again, the two hour period is not narrowly critical, as a greater periodmay be used with impunity, while somewhat lesser periods of time oftenattain a large amount of desirable results sought.

The present invention does not rely upon any particular theory as towhat takes place during strain aging. It

is believed, however, that strain aging is really a diffusion-controlledprocess in which certain solute atoms such as carbon and nitrogenmigrate toward high stress regions created during and as a result ofprestrain. It is further believed that the rate of such migration ordiffusion is approximately doubled for every 10 C. increase intemperature at which the strain aging is conducted.

The foregoing theory, which is believed to be correct but is notspecifically relied upon, tends to explain why strain aging operatesbetter at higher temperature values up to about 50 F. below thetempering temperature and also why it can operate even at roomtemperature, which, for the purpose of the present invention, may beassumed to be 70 F. The preferred range for the strain aging temperatureis about 50 F. to about F. below the tempering temperature and mostpreferably about 50 F. below the tempering temperature.

One result of the prestraining aging as aforesaid is that steel samplesacquire a definite and relatively high yield point as is evidenced froma comparison of curves A and B of FIG. 1 of the drawings wherein thesample tested to give curve A had been tempered in a conventionalmanner, but had not been prestrained and strained aged; while that togive curve B had been tempered, then prestrained and strain aged.

It is recognized that prestraining and strain aging has been describedto some extent in the article hereinabove referred to by Stevenson etal. in the Transactions of the American Society of Metals. In the testsset out in this article, not only was the type of steel inappropriatefor maximum desirable results in accordance with the present inventionin that it had a too low nickel content and a too high chromium content;but also the authors of this article did not know or in any case had notinvestigated and told of the necessity that the strain aging temperatureof retempering, as it was called in that article, should besubstantially below the temperature at which the body was firsttempered. For this reason, therefore, they did not succeed in obtaininga strengthening of the steel to the ranges which they desired, i.e. overabout 300,000 p.s.i., accompanied by reasonable ductility. As aforesaid,they reported a most complete loss of elongation which is of course ameasure of ductility. As compared with this, by the selection of aproper composition, even in the relatively broad range herein set out asappropriate to the present invention, plus the conduct of the strainingand strain aging step at a temperature below that of the originaltempering step, the desired characteristics of ductility are retainedfor the most part, while tremendously improving the strengths of thebodies being produced. While most of the tests hereinafter reported arein tension, and indicate increased strength in tension due toprestraining in tension and subsequent strain aging, similar resultswill be obtained in compression or in right-hand torque or in left-handtorque. Thus, for example, if a body or article is to be used towithstand left-hand torque, it is prestrained in left-hand torque, thenstrain aged so as to give a final product which is improved as to itsability to stand left-hand twisting or torque. This body will not,however, be significantly improved in its resistance to right-handtorque. If, on the other hand, a body is to be subjected to right-handtorque during its normal use, then it is prestrained in right-handtorque and strain aged, whereupon its strength to resist or withstandright-hand torque or twisting is greatly improved, but its ability towithstand left-hand torque or twisting is not significantly improved bythe prestraining and strain aging.

In a similar way, prestraining and strain aging in tension enhances thestrength of a body to withstand tensile forces; whereas prestraining andstrain aging in compression makes a body stronger to withstandcompressive forces; but prestraining in one direction does notsignificantly assist in strengthening the body against forces in the orany other direction. This phenomenon is known as the Bauschinger effectas aforesaid.

9 The present invention will be better understood by a consideration ofa number of actual examples wherein the various embodiments of theinvention will be brought out in detail.

EXAMPLE I low ductility as contrasted with the other samples tested. Itis noted, however, that this sample 2 has the greatest tensile strengthand the highest 0.2% offset yield point. These values, however, wereattained in this particular sample at a substantial cost in theirductility. For these reasons, therefore, the carbon composition ispreferably within the somewhat narrower limits of about 0.45- 0.50%.There is attained, however, even in sample 2 when tempered using arelatively high austenitizing temperature, strength characteristicswhich are beyond those of any other sample tested, so that this sampleis considered broadly to be within the scope of the present invention.

Table 2 EFFECT OF PRESTRAINING AND STRAIN AGING ON THE TENSILEPROPERTIES OF STEELS [0.357-inh Diameter Tensile Test Specimens] Plasticitraln Yield Strength (1,000 p.s.i.) sensilteh fi longattiqn ReductionSample No. Austenitizing Tempering Prestrain ging reng ereen 1n in areaT F. Percent Temp. F.) (1 000 p.s.1.) 2 1n.) (Percent) Temp F) amp 0.02%ofiset 0.2% offset 1, 450 400 2 350 343 352 352 s 33 1, 450 400 3 300344 359 350 4 23 1, 450 400 2 350 339 33s 5 4 1 450 400 2 350 339 353353 s 3 1 450 400 2 300 208 350 350 1 3 29 11525 400 3 350 335 335 1 211 Broke at or outside gage marks.

These steels were formulated and test pieces made there- EXAMPLE IIfrom. Each test piece of steel of samples 1-5 were tempered at 400 F. asset out hereinafter and further subjected to prestraining in tension andstrain aging. The results of these tests are set out in Table II above.

The steels reported on in Table 2 above were, after austenitizing,quenched in molten salt at 500 F. (with the exception of the sample No.PA-l, which was quenched in oil in the conventional manner), held oneminute, and then air cooled. The strain aging when carried on was fortwo hours at the temperature indicated.

The foregoing data is given in full and as the data was taken. It isbelieved, however, that the data as to percent elongation and reductionin area for the sample 2 as austenit-ized at 1550 is erroneous, as thisdata is substantially out of line with the remainder of the samples.Another possible explanation as to this is that this sample had arelatively high carbon content and as such is not within the mostpreferred range of carbon for this general purpose. This may account inpart for the relatively This example is given to illustrate the presentinvention applied. to steels having about 5% nickel. Two actual steelswere made up and tested in accordance with this example, which areherein numbered as samples 6 and 7. The composition of these steels isset out herein below in Table 3.

The balance of these compositions consisted essentially of iron withincidental impurities, including a small amount, less than 0.01% each,of sulfur and phosphorus. These samples were tested as before with theresults set out in Table 4, which follows.

Table 4 RESPONSE OF 5.0% NICKEL STEELS AND THE EFFECT OF STRAIN AGINGTEMPERATURE Plastie Strain Yield Strength (1,000 p.s.i.) TensileElongation Reduction Sample No. Tampering Prostram Aging Temp. Strength(Percent in in area Temp. C F.) (Percent) F.) (1,000 psi.) 2 in.)(percent) 0.02% ofiset Yield point 0.2% offset The table above shows thestrength of the as-tempered bodies made of the respective compositionsand also gives the value for a yield point as such, where the yieldpoint is definite and ascertainable. In several instances duplicatesamples were run under the same conditions which gives a goodapproximation of the extent to which the values may be duplicated inrepeated tests. It will be noted that in each instance there is asubstantial retention of ductility as measured by the elongation andreduction in area, even though the very substantial strengtheningincident to the practice of the process of the present inventionresulted in some loss of ductility. The remaining ductility is adequatefor many and most purposes for which this steel is desired and is muchgreater than was present in prior art steels treated in a somewhatsimilar manner. As to each sample, a set of data is also given where thestrain aging temperature is substantially equal to the temperingtemperature. As will be noted, both the elongation and the reduction inarea and hence the duetility under these conditions are less than in theinstances where the strain aging temperature is 50 F. less than theformer tempering temperature, but such ductility still is substantialand is adequate for most purposes. 0n the other hand, it is consideredundesirable to use a strain aging temperature or retempering temperaturehigher than the original tempering temperature as this has resulted insubstantial decrease in several of the strength characteristics of theproduct as shown by work reported in the prior art publications ofStephenson et a1. above referred to.

EXAMPLE III This example demonstrates principally the desirablecharacteristics of a cobalt and/or aluminum containing steel andpreferably of a steel alloy composition in which both cobalt andaluminum are present. The steels of this character have thecharacteristic of having a relatively high 0.2% offset yield strength(230,000 p.s.i. or more) on an as-tempered basis. In the tests carriedout in determining the characteristics of a number of steels whentempered at various temperatures in the range of about 400800 F., agroup of steels illustrative of different type compositions were used.The compositions of the several samples tested are given in Table 5,which follows.

Table 5 Sample No. 0 M11 Si Ni Cr Mo V Co Al Of the foregoing, sample 8is not the preferred form in accordance with this phase of the inventionas it contains substantial amounts of molybdenum and vanadium; eventhough this steel is broadly within the general scope of this inventionin that it is quite similar to samples 5 and 6 which have quitedesirable properties as to products which have been strained and strainaged. Of the four samples hereinabove given, that designated 10 isapproximately the preferred composition. The foregoing samples weretested on an as-tempered basis with the results set out in Table 6,which follows.

Table 6 [0.357-inc11 round specimens] Yield Strength Elongation (1,000p.s.i.) T605118 Reduction Sample Tampering Strength i area 0. Temp. F.)(1,000 p- (percent) 0.02% 0.2% (Percent in (Percent in offset otlset 2in.) 1 in.)

1 Austenltized at 1,500 I*., quenched into molten salt at 500 F., held 1minute and air cooled.

From the foregoing it will be seen that the compositions of the presentinvention provide quite high 0.2% offset yield points which are,however, substantially below the ultimate tensile strengths in mostinstances, but tend to approach them, particularly as to the compositionof sample 10. It is also clear that increases in tempering temperaturegenerally result in a reduction in tensile strength. However, when thesedata are plotted, it will be noted that the unusual results shown inFIGS. 2 and 3 of the drawings ensue, particularly in that, referring toFIG. 2, there is a peak at 600 F. for 0.2% offset yield point, which ishigher than the corresponding yield point for the same samples temperedat 400 F. In the drawings, curves C and C of FIGS. 2 and 3 are plots ofthe data given as to sample 10; curve D of FIG. 2 and D of FIG. 3 is aplot of the data given for sample 9, which is next preferred; curve E.of FIG. 2 and E of FIG. 3 is a plot of the data given for sample 11;while curve F of FIG. 2 and F of FIG. 3 is a plot of the data for sample8. The dotted line curves of FIGS. 2 and 3 are plots of thecorresponding data for a prior art composition which is generally knownas Type 4150 steel, the composition of which is given hereinabove.

It will also be noted from Table 6 that as to the preferred compoistion,i.e. that of sample 10, there is a second relatively low peak (at240,000 p.s.i.) for the 0.2 offset yield strength versus temperingtemperature at a tempering temperature of 800 F. in addition to the peakat 600 F. shown on FIG. 2.

Another and very impressive demonstration of the particular desirabilityof the composition of sample 10 is the the fact that a 12 inch longbutton-headed tensile bar of this composition having a diameter of 0.419inch (less than inch) when used in a test demonstration supported theweight of a 45,000 lbs. freight car while serving as its sole support bybeing interposed between a rigging secured to the car on the one handand a hook from a heavy crane above the car on the other hand. In thisdemonstration, the test piece was subjected to a stress of 325,000p.s.i. In an earlier test on this same piece, to ascertain whether itcould stand not only the weight of the car, but also the extra stressincident to lifting the freight car several feet off the rails, it wassubjected to a tensile test wherein it sustained a total pull of 50,000lbs., which was equivalent to 360,000 p.s.i. This test piece had beenprepared from the composition of sample 10 by the process herein taughtincluding tempering at 400 F., plastically prestraining the bar about 2%of its original length and strain aging at 350 F. It has been found thatthe 0.2% ofiset yield strength of compositions of this character whentreated in this way is between about 350,000 and 360,000 p.s.i.

Another steel which is similar in its composition to sample 10hereinabove referred to and which is in the class of 3.5% nickel steels,is given as another preferred composition of this type steel. Thiscomposition is:

Carb on0.5 3 Manganese-0.90% Silicon-4.60% Nickel3.60% Chromium-0.30Cobalt2.00 Aluminurn0.75 Phosphorus-0.006% Sulfur0.006

and the balance iron with incidental impurities.

EXAMPLE IV This example is to illustrate. the fracture toughnesscharacteristics of steels having compositions generally in accordancewith the present invention and further to set out preferred compositionsWithin the general scope of the compositions of the present inventionwhich have the best characteristics of fracture toughness.

In order to determine fracture toughness, it is usual at this time todetermine the tensile properties of sharply notched steel sheet. In someinstances the steel sheets used in tests of this kind are notched on thesides, with the notches facing one another and are imperforate in thecenter portion to be tested, i.e. between the ends that are gripped inthe testing apparatus. It is quite common, however, that the endportions of a sample to be gripped in the testing apparatus maybeperforated for a large bar so as to facilitate the gripping of such endportions. ,A discussion of the fracture testing of notched specimens iscontained in the ASTM Bulletins for January and February 1960, pages 29et seq. in the January issue and page 18 et seq. in the February issue.

In the test work done in connection with the present invention, however,a sample as particularly. shown in FIG. 4 of the drawings was used, thisfigure giving exact dimensions of the test piece which included endportions having perforations of 0.500 inch for gripping purposes and acentral hole with the notches disposed on diametrically oppositeportions thereof and extending outwardly from the center hole. Thecentral hole and the notches therein were formed by an electrojetapparatus, i.e. a tool employing an electric arc and operating under anoil coolant. The specimens were then fatigue-cracked at the notchesprior to testing, the notches serving as fatigue crack starters. Thiswas done by repeatedly alternately applying a high tensile stress equalto about one fifth of the yield strength of the material and a lowtensile stress equal to about one-half the high tensile stress forstarting and propagating fatigue cracks. These stresses were appliedintermittently for about 30,000 cycles. This was carried on until thefatigue cracks as shown at 11 and 12, FIG. 4, had a total overall.length as shown by the dimensioins on that figure of about 0.750 inchfrom end to end. In order thereafter to determine the length attained bythe slowly propagating crack at the inception of crack instability,India ink was applied to the notches near the apex of each notch so asto follow up the crack. This length may in turn be used to compute avalue known in the art as fracture toughness. The ink, while wet andfiowable, followed each crack as it was gradually extended up to thepoint where crack instability set in. This technique was used forestimating the length of the slowly propagating crack and from this,fracture toughness values can be computed. This type of testing is thatfavoredv by the ASTM Committee on the Fracturing of High Strength SteelSheet at its Meeting of November 17, 1960.

The composition (in percent by weight) of various steels which have beentested or known data as to which Samples were prepared from theforegoing materials set out in Table 7, the samples being cut in eachinstance first with the long axes of the samples, i.e. the verticaldirection of the drawing as seen in FIG. 4, extending longitudinally ofthe roll sheet (i.e. longitudinally of the direction of rolling). Ineach instance the sample was cut out from the sheet and the holes andnotches formed therein prior to conventional austenitizing, quenchingand tempering, all of which was then carried on with the formed andpunched sheet and' then the fatigue cracks formed therein by thealternate application of high and low loads in a longitudinal direction.of the sa'mple,.the

high load being about twice the low load. It is found that the amount ofloading and the number of cycles required to form such fatigue cracksare a function of the frequency of the alternationbetween high and lowloading and hence the data as to the manner as to which these testpieces was fatigue-cracked is not per se characteristic. Suflice it tosay that the fatigue cracks plus the. center hole diameter and thedepths of the notches extended initially a distance of about 0.75 inchas set out in the drawing, FIG. 4. The samples were then tested at roomtemperature in tension and the maximum tension determined up to thepoint of crack instability. The length of the crack at the point ofcrack instability was also determined by the ink technique as abovedescribed, as the ink follows the crack propagation up to the point ofcrack instability. The net notch strength for each sample determinedfrom the above test was calculated as the maximum applied load dividedby the uncracked cross section area of the sample at the onset ofunstable crack propagation.

As so tested, sample 13, which is substantially the preferredcomposition in accordance with the present invention for resistanceagainst crack propagation or fracture toughness as it is sometimescalled, showed a net notch strength averaging 246,000 p.s.i. for anaverage of three specimens tested, the actual values for these specimensbeing 230,000, 248,000 and 261,000 and a fourth specimen also testedgiving the value of 258,000. These specimens were tempered at 400 andhad varying thickness between 0.096 inch for the first three in questionand 0.116 inch for the sample having the 258,000 p.s.i. test results. Ineach instance the initial crack length was between 0.750 inch and 0.780inch and the final crack length also varied from 1.17 inch to 1.27 inch.These figures for notched tensile strength may be compared with the 0.2offset yield strength of smooth bar test pieces of the same compositionof about 210,000-220,000 p.s.i. and a total tensile strength of about260,000 p.s.i., also for smooth round bar test piece.

These results may further be compared with that of the prior artcomposition hereinabove given as sample PA-3 wherein a similar testpiece showed a notched tensile strength of only 189,000 p.s.i. Again,sample PA-S, which is a steel of the 4340 type, averaged about 206,000p.s.i. for net notch strength (calculated as aforesaid), with ditferentsamples tested giving various values between 202,000 and 209,000 p.s.i.The net notch strength values of the steel according to the presentinvention may also be compared with the figures ascertainable from priorart literaturev as to the composition given as PA-6, which was the bestof the prior art samples and had a net notch tensile strength of 218,000p.s.i.

The characteristics of the failure of the several samples is also ofgreat interest. Those having compositions according to the presentinvention, when cut as longitudinally extending samples, were 100% shearfailures, which is the desired condition. As contrasted with this, thesamples of the several prior art compositions were, in practically allinstances, substantially less than 100% in shear, values of 73%, 80% and95% being common for different samples.

7 Some samples of the preferred composition were also treated in thefollowing manner, sometimes known as hot-cold working. This involvedaustenitizing the material in sheet form at 1450 F., then quenching inmolten salt at 600 F. and immediately thereafter and prior to thecooling of the sample, rolling in a single roll pass so as to reduce thethickness of the sheet by 35%, followed by quenching in oil at about 120F. The samples as thus prepared were then tested to give relatively lownet notch tensile strength results of about 87,000 p.s.i. When, however,samples prepared in this way up to the point of testing were furthertempered at 400 F., (all these samples in accordance with the presentinvention having a thickness between 0.054 and 0.060 inch) the testresults showed 1 1 notch tensile strength values of 15 239,000 and262,000 p.s.i. with final cracked lengths of 1.30 inch and 1.32 inchrespectively as against final crack lengths for the first group ofsamples having net notch strengths of 87,000 p.s.i. of only 0.82 and0.87 inch respectively.

The importance of steel capable of withstanding notchtype tests asaforesaid is believed to be very great in that such cracks have a verysmall root radius, of 0.001 inch or less. Cracks of this nature could beformed in the manufacture of many articles from sheet steel where it isdesired that the article shall have great strength and shall not failunder extreme operating conditions. Such cracks could be generatedduring heat treating or welding or other fabrication operations in thecourse of manufacturing articles from sheet steel. Cranks of this kindmay not be readily detectable, even if detectable at all, during normalinspection. For these reasons, therefore, resistance to crackpropagation is often considered of greater importance than mere tensilestrength values of the steel.

Samples 14 and 15 above referred to were tested at both 400 and 600 F.tempering temperatures. The tests made at 400 showed that a sample ofthe type shown in FIG. 4 and 0.087 inch in thickness had a 100% sheartype failure and had a net notch strength as hereinabove defined of250,000 p.s.i. Sample 15, also tempered at 400 F., was similarly testedwith the sample formed as shown in FIG. 4 and with a thickness of 0.086inch. The failure in this instance was only 58% of a shear type and thenet notch strength only 155,000 p.s.i. This indicates that cobalt at the2% level is completely tolerable. However, the carbon-cobalt balance insample No. 15 is somewhat high and consequently samples of thisthickness range did not break with full shear failures and high netnotch strengths.

When samples 14 and 15, each of a size and configuration as shown inFIG. 4 and 0086-0087 inch in thickness, were tempered at 600 F., thesamples broke with full shear failures and high net notch strengths(about 250,000-256,000 p.s.i.). This indicates that at the highercobalt-carbon balance and at the higher tempering temperatures, fullshear failures and high net notch strengths may be achieved, even at theaforementioned thickness. The compositions of samples 14 and 15, whenformed into smooth, round tensile bars 0.357 inch in diameter andtempered at 600 F., when tested at room temperature, had 0.2 offsetyield strengths of about 235,000- 237,000 p.s.i. and tensile strengthsof about 255,000- 257,000 p.s.i. It is thus shown that all the net notchstrengths for the 600 F. tempered samples Nos. 14 and 15 weresubstantially in excess of the smooth bar yield strengths.

Another steel alloy composition which has been shown to have good notchproperties, in that it is highly resistant to crack propagation .of anotched sample as aforesaid, is one having 0.48% carbon, 0.45%manganese, 0.25% silicon, 5.10% nickel, 0.30% chromium, 0.30%molybdenum, 0.12% vanadium and 0.006% (max) each of sulfur andphosphorus with the balance iron with incidental impurities.

While there has been disclosed herein a relatively broad range of mediumcarbon alloy steels in accordance with the present invention, certainmore limited ranges for particular purposes, and specific compositionsalso for particular purposes, all as set out herein, and while a processof treating steel to attain super strengths in a particular directionhas also been disclosed, other variations and equivalents of theforegoing will become evident to those skilled in the art based uponthis disclosure and the appended claims. I do not intend to be limited,

' therefore, except by the scope of the appended claims,

which are to be construed validly as broadly as the state of the artpermits.

17 What is claimed is: 1. A super strength steel alloy composition,consisting essentially of the following:

About 0.350.60% carbon About 3-7% nickel About 0.2-0.5% chromium About2% manganese About 0-2% silicon About 00.5% molybdenum About 00.2%vanadium About 0-5% cobalt not over about 0.01% each of sulfur andphosphorus, and the remainder being iron with incidental impurities; themetal being characterized by high as-tempered strength accompanied bygood ductility.

2. A super strength steel alloy composition in accordance with claim 1,in which the composition contains about 3 /2% to nickel.

3. A super strength steel alloy composition in accordance with claim 1,in which the composition contains about 0.45 to 0.50% carbon.

4. A super strength steel alloy composition in accordance with claim 1,in which the composition contains about 3%. to 5% nickel and about 0.45to 0.50% carbon.

5. A super strength steel alloy composition in accordance with claim 1,in which the composition contains About OAS-0.55% carbon, About 3 /2-4%nickel, About 0.2-1.2% manganese, About 0.2-0.4% chromium, About 1.7-2%silicon, and

substantially no molybdenum or vanadium, the composition being furthercharacterized by very high strength when austenitized, quenched,tempered, prestrained beyond its elastic limit in the direction in whichits super strength is to be utilized and to a permanent strain of about1 to about 5% beyond its original dimensions and thereafter strain agedat a predetermined elevated temperature which is not substantially morethan the tempering temperature thereof.

6. A super strength steel alloy composition in accordance with claim 1,in which the composition contains About 04-05% carbon, About 5% nickel,

About 0.4-0.9- manganese, About 0.15-l.1% silicon, About OBS-0.4%chromium, About 0.14% vanadium, and About 0.3-0.35% molybdenum;

the composition being further characterized by very high strength whenaustenitized, quenched, tempered, prestrained beyond its elastic limitin the direction in which its super strength is to be utilized and to apermanent strain of about 1 to about 5% beyond its original dimensions,and thereafter strain aged at a predetermined temperature which is notsubstantially above the tempering temperature thereof.

7. A super strength steel alloy composition in accordance with claim 1,in which the composition contains About 0.35-0.5% carbon,

About 4-6% nickel,

About 0.2-0.5% chromium,

About 0.3-1% manganese,

About 0.2-1% silicon,

About 0.15-0.5 molybdenum, and About 0.5-0.3 vanadium;

the composition being further characterized by high fracture toughnessas measured by the resistance of a notched sample of sheet material tocrack propagation under tensile stress.

18' 8. A super strength steel alloy composition in accordance with claim1, in which the composition contains Abuot OAS-0.5% carbon,

About 4.8-5.5 nickel,

About 0.2-0.4% chromium, About OAS-0.9% manganese, About 0.2-0.9%silicon,

About 0.2-0.4% molybdenum, and About 0.10.15% vanadium;

the composition being further characterized by high fracture toughnessas measured by the resistance of a notched sample of sheet material tocrack propagation under tensile stress.

9. A super strength steel alloy composition characterized by highfracture toughness as measured by the resistance of a notched sample ofsheet to crack propagation under tensile stress, said compositionconsisting essentially of the following:

About 0.48% carbon, About 5.10% nickel, About 0.30% chromium, About0.45% manganese, About 0.25% silicon, About 0.30% molybdenum, About0.12% vanadium,

and not over about 0.006% each of sulfur and phosphorus. 10. A superstrength steel alloy composition in accordance with clam 1, in which thecomposition contains:

About 0.40.6% carbon About 3-6% nickel About 0.2-0.4% chromium About03-13% manganese About 1-1.8% silicon About 1-3 /z cobalt, and About0.2-1% aluminum;

said composition beingcharacterized in that as-ternpered articles madethereform have high 0.2 offset yield strengths and high tensilestrengths, with excellent ductility over a broad range of temperingtemperatures from about 400 F. to about 900 F.

11. A super strength steel alloy composition in accordance with claim 1,in which the composition contains:

About OAS-0.55% carbon About 3.5-4.0% nickel About 0.2-0.4% chromiumAbout 0.6-1.0% manganese About 1.4-1.8% silicon About 1.5-2.5 cobalt,and About 0.6-0.8% aluminum;

About 0.53% carbon About 3.60% nickel About 0.30% chromium About 0.90%manganese About 1.60% silicon About 2.00% cobalt About 0.75% aluminumand not over about 0.006% each of sulfur and phosphorus and theremainder being iron with incidental impurities.

(References on following page) 19 References Cited by the Examiner2,715,576 2,791,500 UNITED STATES PATENTS 2,879,194 6/20 Johnson75128.85 2,978,319 8/43 Bagsar 75128.0 5 6/46 Mohling 148-136 12/50Zikmund 75-128 20 V Payson et a1. 75124 Foley 75-124 Eichelberger 148136Chang 75-128 DAVID L. RECK, Primary Examiner.

MARCUS U. LYONS, Examiner.

1. A SUPER STRENGTH STEEL ALLOY COMPOSITION, CONSISTING ESSENTIALLY OFTHE FOLLOWING: